fix: use composition.reg_id for entity identity in slot_of

get_children() returns fresh userdata wrappers without __eq; raw ==
compares Lua pointers (fails across separate get_children calls).
slot_of now reads composition.reg_id from each child to find the
matching slot, making contains/remove reliable across wrapper instances.

init.lua

@@ -36,11 +36,26 @@ local function next_slot(container)
     return string.format("item.%d", max_n + 1)
 end
 
+-- Return a stable entity identity key for comparison.
+-- Uses composition.reg_id (a monotonic integer set on all composition
+-- entities). Falls back to nil for raw entities.
+local function entity_id(e)
+    local rid = e:get_property("composition.reg_id")
+    -- reg_id is 0 for non-composition entities (or unset); treat as nil.
+    if rid and rid ~= 0 then return rid end
+    return nil
+end
+
 -- Find the slot name under which `item` is attached to `container`.
 -- Returns the slot string or nil if not found.
+-- Compares by composition.reg_id because get_children() returns fresh
+-- userdata wrappers (no __eq on entity userdata — raw pointer differs).
 local function slot_of(container, item)
+    local item_rid = entity_id(item)
+    -- get_children() returns fresh userdata wrappers each call; no cache.
     for slot_name, child in pairs(container:get_children()) do
-        if child == item then
+        local child_rid = entity_id(child)
+        if item_rid ~= nil and child_rid == item_rid then
             return slot_name
         end
     end
@@ -55,14 +70,18 @@ local function validate_container(container)
         error("inventory.add: entity has no container block " ..
               "(template must declare container={kind='list'})")
     end
+    -- Defensive: composition.define_template currently rejects non-list
+    -- kinds at template-load, so this branch is unreachable via normal flow.
+    -- Kept as forward-protection if composition relaxes that constraint.
     if block.kind ~= "list" then
         error(string.format(
             "inventory.add: container.kind must be 'list' (got '%s')",
             tostring(block.kind)))
     end
-    -- Double-belt: reject any declared constraint fields (composition v0.2
-    -- already rejects these at template-load, but we guard here too so the
-    -- lib is safe against containers created outside the composition gateway).
+    -- Defensive: composition v0.2 rejects declared constraint fields at
+    -- template-load (weight_max/volume_max/grid/accepts_fluid/accepts_gas/
+    -- restrictions), so this branch is unreachable via normal flow. Kept
+    -- as forward-protection if composition relaxes that constraint.
     local constraint_fields = {
         "weight_max", "volume_max", "grid",
         "accepts_fluid", "accepts_gas", "restrictions",
@@ -159,12 +178,12 @@ function M.contents(container)
     return out
 end
 
--- contains(container, item) -> bool
+-- Returns true iff `item` is currently in `container`.
 function M.contains(container, item)
     return slot_of(container, item) ~= nil
 end
 
--- count(container) -> number
+-- Returns the number of items currently in `container`.
 function M.count(container)
     return #M.contents(container)
 end